In recent years, smaller and lighter mobile data terminals such as mobile phones, notebook personal computers, and smartphones have been increasingly used and batteries used as driving power supplies therefor have been required to have higher capacity. Nonaqueous electrolyte secondary batteries, which are charged and discharged in such a manner that lithium ions move between positive and negative electrodes in association with charge and discharge, have high energy density and high capacity and therefore are widely used as driving power supplies for the above mobile data terminals.
Furthermore, the nonaqueous electrolyte secondary batteries are recently attracting attention as utility power supplies for electric tools, electric vehicles, and the like and applications thereof are expected to be further expanded. In such a field, high capacity and excellent power characteristics are required.
The use of, for example, a silicon-containing material as a negative electrode active material for the nonaqueous electrolyte secondary batteries is under investigation. For example, Patent Literature 1 below discloses a negative electrode active material composed of cores containing a compound (where the atomic ratio x of O to the total of Si and Sn is given by 0.5≤x≤1.5) containing Si or Sn and O as constituent elements and carbon cover layers covering the surfaces of the cores. It is suggested that a negative electrode can be formed by applying a negative electrode mix paste obtained by kneading a solvent and a mixture containing the negative electrode active material, a binder (binding agent), and the like to a current collector and a conductive aid may be further added to the mixture.
However, in the case of using a silicon-containing material as an active material, although high capacity is expected, there is a problem that the capacity is reduced because the active material significantly expands and contracts in association with the storage and release of lithium ions during charge and discharge and therefore the adhesion between an active material layer and a current collector is reduced by the pulverization of the active material. In order to improve the adhesion between an active material layer and a current collector, Patent Literature 2 below proposes that the active material layer is configured such that the amount of a binding agent decreases from the interface between the active material layer and the current collector toward the outside.